Serious eye disease can increase the risk of sleep disorders in teenagers, particularly if it involves the optic nerve that carries visual signals to the brain.

"We suspect these patients have difficulty using daylight to synchronize their internal rhythms to the outside world," says Dr. Russell Van Gelder, a researcher at Washington University in St. Louis.

Van Gelder and his team studied 25 students, ages 12 to 20, from the Missouri School for the Blind and 12 students with normal sight from a suburban St. Louis school.

The visually impaired students were divided into two groups: those whose visual problems were related to optic nerve disease and those whose vision loss did not involve the optic nerve. All participants wore a watch-like device for two weeks, which enabled the researchers to monitor their biological clock, or circadian rhythms.

Participants with optic nerve disease were 20 times more likely to be "pathologically sleepy" (napping 20 or more minutes per day) than those with normal sight. They also were nine times more likely to have pathologic sleepiness than children who were blind from non-optic nerve diseases.